MessagePort

The MessagePort interface of the Channel Messaging API represents one of the two ports of a MessageChannel, allowing sending of messages from one port and listening out for them arriving at the other.

Note: This feature is available in Web Workers.

Methods

Inherits methods from its parent, EventTarget

MessagePort.postMessage
Sends a message from the port, and optionally, transfers ownership of objects to other browsing contexts.
MessagePort.start
Starts the sending of messages queued on the port (only needed when using EventTarget.addEventListener; it is implied when using MessagePort.onmessage.)
MessagePort.close
Disconnects the port, so it is no longer active.

Event handlers

Inherits event handlers from its parent, EventTarget

MessagePort.onmessage
Is an EventListener that is called whenever an MessageEvent of type message is fired on the port — that is, when the port receives a message.

Example

In the following code block, you can see a new channel being created using the MessageChannel.MessageChannel constructor. When the IFrame has loaded, we transfer MessageChannel.port2 (a MessagePort) to the IFrame using MessagePort.postMessage along with a message. The handleMessage handler then responds to a message being sent back from the IFrame (using MessagePort.onmessage), putting it into a paragraph.

var channel = new MessageChannel();
var para = document.querySelector('p');
    
var ifr = document.querySelector('iframe');
var otherWindow = ifr.contentWindow;

ifr.addEventListener("load", iframeLoaded, false);
    
function iframeLoaded() {
  otherWindow.postMessage('Hello from the main page!', '*', [channel.port2]);
}

channel.port1.onmessage = handleMessage;
function handleMessage(e) {
  para.innerHTML = e.data;
}   

For a full working example, see our channel messaging basic demo on Github (run it live too).

Specifications

Specification Status Comment
WHATWG HTML Living Standard
The definition of 'MessagePort' in that specification.
Living Standard No difference from HTML5 Web Messaging.
HTML5 Web Messaging
The definition of 'MessagePort' in that specification.
Recommendation W3C version of the spec

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 4 Not supported 10.0 10.6 5
Available in workers (Yes) 41 (41) (Yes) (Yes) (Yes)
Feature Android Chrome Mobile Firefox Mobile (Gecko) Firefox OS (Gecko) IE Phone Opera Mobile Safari Mobile
Basic support 4.4 4 Not supported Not supported 10.0 11.5 5.1
Available in workers (Yes) (Yes) 41.0 (41) (Yes) (Yes) (Yes) (Yes)

See also

Using channel messaging

Document Tags and Contributors

Contributors to this page: chrisdavidmills, baku, fscholz, kscarfone
Last updated by: chrisdavidmills,
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